This invention relates to a commercial process for the production of sulfates from by-product aluminum oxides.
Commercially produced aluminum is generally derived from two sources. Primary aluminum is obtained from smelting alumina. This process and subsequent remelting and casing operation results in the production of drosses and other by-products which remain high in aluminum content and are a source of secondary aluminum. Processing for recovery of secondary aluminum results in by-product aluminum oxides.
The disposal of these aluminum oxides is a problem of increasing magnitude to the aluminum industry. Usually the aluminum oxide wastes are stockpiled causing mountains of environmental blight and it is an object of this invention to provide a commercially feasible method for disposal of aluminum oxide by-products.
Aluminum sulfates may be produced by the reaction of aluminum oxide and sulfuric acid. The reaction has the equation: EQU Al.sub.2 O.sub.3 +3H.sub.2 SO.sub.4 .fwdarw.Al.sub.2 (SO.sub.4).sub.3 +3H.sub.2 O
In practice it is the hydrated form (the aluminum oxide molecule chemically bonded with one or three molecules of water) that will react with the acid.
Aluminum sulfate compounds have utility in both the production of paper products and the water treatment industry. In the latter industry liquid aluminum sulfate is used as a flocculant and as a clarifier, while in the paper industry it is used to set the sizing agent. Up to now, such high grade aluminum sulfate was derived from commercial bauxite and Arkansas clay which has a high aluminum oxide content. It is, therefore, another object of this invention to conserve natural resources by providing a method for the production of aluminum sulfate of high purity for use in papermaking and water treatment from by-product oxide wastes.
A typical by-product oxide composition would probably contain the following elements in a physical/chemical matrix:
______________________________________ Element Wt % ______________________________________ Al 25-50 Si 0.5-2.0 Mg 3-7 Fe 0.05-2.0 Ca 0.3-1.0 Ti 0.06 Zn 0.1-1.0 N 0.5-5.0 C 0.03-0.3 Na 0.5-2.0 Cu 0.1-0.5 Mn 0.1-0.2 K 0.3-0.7 Cl 1.0-3.7 ______________________________________
When sulfuric acid is combined with such a complex by-product composition, there will be a multitude of reactions, some of which will produce sulfates other than aluminum, unless the reaction conditions are strictly controlled. Some of the oxide components will be unreactable.
The major proportion of aluminum available for reaction is found in its oxide form. Under the conditions described in this invention, the hydrated portion of the oxide will react with sulfuric acid according to the previously mentioned equation.
Some of the aluminum is present in the by-product matrix in zero valent form. This elemental form has been found to react with sulfuric acid to form aluminum sulfate, but hydrogen gas, which would be expected as a reaction product if pure metallic aluminum were present, has not been detected in significant quantity. On occasion significant sulfur dioxide has been detected as a reaction product. This latter condition has been reported in the literature according to the equation. ##EQU1##
Aluminum carbide reacts to produce sulfate according to the equation: EQU Al.sub.4 C.sub.3 +6H.sub.2 SO.sub.4 .fwdarw.3CH.sub.4 +2Al.sub.2 (SO.sub.4).sub.3
Another reaction which has great commercial utility is that of aluminum nitride. However, the principal object of this invention is the production of aluminum sulfate for use in water treatment and in papermaking which contains reaction products and by-product compounds which are not detrimental to these industries. It was determined that the inclusion of significant amounts of Fe, Mg, Si, Na and K in the aluminum sulfate product does not adversely affect the product when used in the above-recited industries. Inclusion of significant quantities of Cu, Cr, Mn and Pb would, however, effectively remove the reaction product for use in water treatment and papermaking. Thus, the processing steps as described below recite those conditions which convert significant quantities of aluminum sulfate from by-product oxide wastes without the inclusion of toxic heavy metals, such as Cu, Cr, Mn and Pb.
When the aluminum oxide by-products have a high nitride content, the process of this invention can be utilized to produce the binary salt, aluminum ammonium sulfate. The reaction follows the equation: EQU 2 AlN+4H.sub.2 SO.sub.4 .fwdarw.Al.sub.2 (SO.sub.4).sub.3 .multidot.(NH.sub.4).sub.2 SO.sub.4
Therefore, it is another object of this invention to provide a process to maximize the recovery of the aluminum in the by-product wastes in the form of aluminum sulfate and aluminum sulfate compounds.